Internal combustion engines exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds such as nitrogen oxides (NOX), and solid particulate matter also known as soot. Due to increased environmental awareness, exhaust emission standards have become more stringent, and the amount of NOX and soot emitted to the atmosphere by an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
In order to ensure compliance with the regulation of NOX, a strategy called selective catalytic reduction (SCR) for treating the exhaust gas can be implemented. SCR is a process where a gaseous or liquid reductant, e.g. ammonia, urea or a urea solution, is injected into the exhaust gas stream of an engine. The reductant reacts with nitrogen oxides in the exhaust gas to form water and nitrogen. Usually, urea is introduced into the exhaust gas in an amount sufficient to provide the degree of NOX-reduction desired. The desired amount of the reductant can be controlled by, e.g., a urea injection system.
Generally, SCR can be effective, e.g., within a temperature range from about 200° C. to about 500° C. Through providing a catalytic surface in the form of the SCR catalyst, the SCR process can be promoted and more efficiently performed, in particular, at lower temperatures. Thus, it is important to provide a suitable temperature and a suitable catalytic material with a sufficiently large surface for the SCR reaction.
SCR as used herein generally includes those processes that utilize ammonia or any NOX-reducing reagent capable of generating ammonia gas upon heating. The term “urea” as used herein is further meant to comprise all those reductants that are commercially available for performing SCR.
An SCR emission control system is disclosed, for example, in WO 2009/017598 A1 and a temperature-adjusted SCR system is disclosed, for example, in WO 2008/054631 A1. Moreover, DE 10 2008 017 280 A1 discloses an arrangement of at least one catalyst and/or particulate filter within a two-stage turbocharged system. Moreover, DE 10 2008 061 222 A1 discloses a multi-stage charged combustion engine having an SCR catalyst arranged between a high-pressure turbine and a low-pressure turbine, wherein the housing of the low-pressure turbine and the housing of the catalyst are mounted to the engine.
EP 1 691 046 A1 discloses a temperature maintenance device for maintaining the temperature of a reducing agent in an exhaust gas clarification apparatus for an engine.
As an alternative for SCR, exhaust gas recycling (EGR) is used to reduce NOX emission. The not yet published EP application 09002111 (filed by Caterpillar Motoren GmbH & Co. KG on 16 Feb. 2009) discloses a turbocharged engine with exhaust gas recycling. In particular, the EP application discloses an engine having end sides being opposite in a lengthwise direction of a crankshaft. On each side, a single-stage turbocharger system, i.e. an exhaust turbocharger with a turbine and a compressor, is arranged. The turbines are fluidly connected to a common exhaust manifold of the engine, while the compressors are fluidly connected to a common intake manifold of the engine.
For medium speed internal combustion engines, two-stage turbocharged systems having a low-pressure stage turbocharger and high-pressure stage turbocharger can be used to pre-compress the charge air being provided to the combustion chambers. The turbocharged systems can be mounted to one side of an engine block and, accordingly, can be exposed to vibrations caused by the operating engine. Usually, the low-pressure stage turbocharger of a two-stage turbocharged system may be exposed to an increased oscillation amplitude if it is mounted at a relatively large distance via the high-pressure stage turbocharger to the engine.
Furthermore, oxidation catalysts are known for exhaust gas treatment, e.g., of diesel and natural gas engines.
In addition, in order to provide a sufficiently large surface for the catalytic reaction, the size of the, e.g., SCR catalyst can increase with the amount of exhaust gas generated, e.g., for medium speed internal combustion engines.
The present disclosure is directed, at least in part, to improving or overcoming one or more aspects of prior systems.